Plant stable isotope ratios (¹⁵N) as an indicator of waste water nitrogen uptake

Abstract

An investigation was conducted into the processes that result in waste water N enrichment in ¹⁵N and also the use of this phenomenon in tracing the source of plant N in the environment. A study of ¹⁵N enrichment in various stages of a treatment plant was carried out at the Horotiu treatment plant with the Upper Waikato River and Northern Manukau Harbour systems being used in tracer studies in the environment. Waste water NH₄ in the Horotiu treatment plant was found to become progressively enriched as it passes through the various treatment systems. The inorganic NH₄ in the raw dissolved raw effluent has been shown to be enriched by +4.1 ⁰/₀₀. In the anaerobic pond organic nitrogen is mineralised and mixes with the inorganic NH₄ resulting in an increase in enrichment to +10.7 ⁰/₀₀ due to the mass balance of the two materials. In the facultative pond nitrification and possibly NH₃ volatilisation further enriches the NH₄ to +13.3 ⁰/₀₀. In the Upper Waikato River Lagarosiphon major and Enteromorpha nana were used in a ¹⁵N tracer study of the source of plant N due to their presence in both natural and waste water contaminated sections of the river. Ammonium is the major source of N, available to the two plants and is derived from treated waste water and Lake Taupo NH₄. The ¹⁵N abundance of waste water was +7.9 ⁰/₀₀ enriched, while the Lake Taupo source was estimated using the values for a range of upstream plants to be near atmospheric natural abundance (ie. 0.0 ⁰/₀₀). Isotopic fractionation during the assimilation of ammonium in the river appears to be low, this has enabled plant ¹⁵N abundance to be used as indicator of source mixing. A two source mass balance mixing model has been used to determine the origin of N in the two species and indicates that 50 to 100 % of plant nitrogen is from waste water. Treated waste water ammonium has previously been found to be naturally enriched in ¹⁵N, an investigation was conducted to identify the cause of this enrichment and it’s potential use as a natural tracer of biological processing. A study was carried out in the Horotiu Treatment Plant to examine the ¹⁵N enrichment of waste water NH₄ in various systems and isotopic fractionation during the processing of nitrogen. The Upper Waikato River and Northern Manukau Harbour were used in studies investigating the impact of enriched waste water NH₄ on plant N isotopic ratios in the environment. Waste water NH₄ in the Horotiu Treatment Plant was found to become progressively more enriched as it passes through various treatment systems, this has been attributed to the enriched ¹⁵N abundance of the substrate and various processes that interact with N at each level. Raw waste water from the nearby Affcco freezing works enters the treatment plant with a low NH₄ concentration (12.7 mgl⁻¹) derived primarily from blood and faeces and has a ¹⁵N enrichment of +4.1 ⁰/₀₀. In the anaerobic pond protein is mineralised and mixes with the existing NH₄ to produce a much higher NH₄ concentration (106 mgl⁻¹) without significant isotope fractionation, so that the waste water NH₄ enrichment now resembles the value of the new substrate (ie.+10.7 ⁰/₀₀). In the facultative pond nitrification and possibly NH₃ volatilisation further enrich ¹⁵N abundance of NH₄ to +13.3 ⁰/₀₀, this is the level of enrichment in waste water that is discharged from the treatment plant. In the Upper Waikato River Lagarosiphon major and Enteromorpha nana were used in a ¹⁵N tracer study to determine the source of plant nitrogen downstream of the Taupo Borough Pollution Control Plant, discharge into the river. Treated waste water and Lake Taupo, are the major sources of N in the river, with NH₄ being the predominant form available to plants. The ¹⁵N abundance of waste water NH₄ was found to be +7.9 ⁰/₀₀ enriched, while the isotopic ratio of Lake Taupo source was estimated from the range of isotopic values found in upstream plants to be near 0.0 ⁰/₀₀. Isotopic fractionation during assimilation appears to be small, this has enabled the use of a two source mixing model, using the ¹⁵N abundance values from plants and the two N sources, to show that 50 to 100% of plant nitrogen in the downstream section of the river is from waste water (depending on the site). In the Northern Manukau Harbour an attempt was made to apply ¹⁵N methodology to tracing the impact of waste water NH₄ (the predominant source of NH₄ in the harbour) on plant nitrogen. Ammonium is the major form of N assimilated by plants in the northern harbour. Sediment and plant N (from Gracillaria) indicated that there is a significant fractionation during assimilation, which highest near the waste water outfalls from the Manukau treatment plant, where plant N is -1 to +9 ⁰/₀₀ enriched compared with NH₄ c.+20 ⁰/₀₀. The degree of fractionation indicates that only a small portion of the NH₄ in the harbour is assimilated. A study of SOM indicated that ¹⁵N isotopic ratios in the treatment plant were different to the values in the southern harbour (ie. c. -5 ⁰/₀₀ compared with +12 ⁰/₀₀). A mass balance model was used to indicate the mixing of the two sources of SOM in the harbour. Results show that c. 30% of the SOM in the northern harbour is derived from the treatment plant, decreasing to 7% in the harbour mouth where a further mixing with SOM from the southern harbour is apparent.